Neuronal cell types in entorhinal cortex and hippocampal formation of man and other mammalia: An interspecies comparison

New paradigm to assess brain cell morphology by diffusion-weighted MR spectroscopy in vivo

The brain is one of the most complex organs, and tools are lacking to assess its cellular morphology in vivo. Here we combine original diffusion-weighted magnetic resonance (MR) spectroscopy acquisition and novel modeling strategies to explore the possibility of quantifying brain cell morphology noninvasively. First, the diffusion of cell-specific metabolites is measured at ultra-long diffusion times in the rodent and primate brain in vivo to observe how cell long-range morphology constrains metabolite diffusion. Massive simulations of particles diffusing in synthetic cells parameterized by morphometric statistics are then iterated to fit experimental data. This method yields synthetic cells (tentatively neurons and astrocytes) that exhibit striking qualitative and quantitative similarities with histology (e.g., using Sholl analysis). With our approach, we measure major interspecies difference regarding astrocytes, whereas dendritic organization appears better conserved throughout species. This work suggests that the time dependence of metabolite diffusion coefficient allows distinguishing and quantitatively characterizing brain cell morphologies noninvasively.

Cytoarchitectonic organization of the entorhinal cortex of the canine brain

The present study describes the cytoarchitectonic and chemoarchitectonic organization of the canine entorhinal cortex (EC). We distinguished medial, laterodorsal, and latero-intermediate subdivisions based on the organization of cortical layers using Nissl and Timm staining and AChE histochemistry. The medial subdivision is located at the border of the parasubiculum and is characterized by a narrow cortex, wide layer II, and densely packed cells in layer V. At its caudal extent, distinct spherical groups of small cells are situated at the border of layer I/II. The laterodorsal subdivision is located along the rhinal sulcus and borders area 35 of the perirhinal cortex. Its cortex is wide and layers tend to merge. Layer II of the laterodorsal subdivision contains scattered "stellate" cells, which are not organized into islands. The latero-intermediate subdivision displays a complex layer organization. The most easily distinguished is layer II, which is comprised of two main cell populations; "stellate" neurons arranged into "islands" and small, round cells distributed within and below the stellate cells. Layer III contains sparse cells that are arranged into vertical clusters, whereas layer IV (lamina dissecans) is especially wide. Nine fields, named according to their rostral to caudal position, were distinguished based on further analyses of layer differentiation. The main features of the rostrocaudal differentiation are a gradual disappearance of "island" organization in layer II, increasing cortical thickness, and wider layers containing small and more densely packed cells. Cytoarchitectonic differentiation was determined by observation of specific histochemical patterns of AChE- and Timm-stained sections.

Dendritic morphology, local circuitry, and intrinsic electrophysiology of principal neurons in the entorhinal cortex of macaque monkeys

Little is known about the neuroanatomical or electrophysiological properties of individual neurons in the primate entorhinal cortex. We have used intracellular recording and biocytin-labeling techniques in the entorhinal slice preparation from macaque monkeys to investigate the morphology and intrinsic electrophysiology of principal neurons. These neurons have previously been studied most extensively in rats. In monkeys, layer II neurons are usually stellate cells, as in rats, but they occasionally have a pyramidal shape. They tend to discharge trains, not bursts, of action potentials, and some display subthreshold membrane potential oscillations. Layer III neurons are pyramidal, and they do not appear to display membrane potential oscillations. The distribution of dendrites and of axon collaterals suggests that neurons in layers II and III are interconnected by a network of associational fibers. Layer V and VI neurons are pyramidal and tend to discharge trains of action potentials. The distribution of dendrites and axon collaterals suggests that there is an associative network of principal neurons in layers V and VI, and they also project axon collaterals toward superficial layers. Importantly, entorhinal cortical neurons in monkeys appear to exhibit significant differences from those in rats. Morphologically, neurons in monkey entorhinal layers II and III have more primary dendrites, more dendritic branches, and greater total dendritic length than in rats. Electrophysiologically, layer II neurons in monkeys exhibit less sag, and subthreshold oscillations are less robust and slower. Some monkey layer III neurons discharge bursts of action potentials that are not found in rats. The interspecies differences revealed by this study may influence information processing and pathophysiological processes in the primate entorhinal cortex. J. Comp. Neurol. 470:317-329, 2004.

Hippocampal fields in the hedgehog tenrec. Their architecture and major intrinsic connections

The Madagascan lesser hedgehog tenrec was investigated to get insight into the areal evolution of the hippocampal formation in mammals with poorly differentiated brains. The hippocampal subdivisions were analyzed using cyto- and chemoarchitectural criteria; long associational and commissural connections were demonstrated with tracer techniques. The hedgehog tenrec shows a well differentiated dentate gyrus, CA3 and CA1. Their major intrinsic connections lie within the band of variations known from other species. The dentate hilar region shows calretinin-positive mossy cells with extensive projections to the molecular layer. The calbindin- and enkephalin-positive granule mossy fibers form a distinct endbulb and do not invade the CA1 as reported in the erinaceous hedgehog. Isolated granule cells with basal dendrites were also noted. A CA2 region is hard to identify architecturally; its presence is suggested due to its contralateral connections. Subicular and perisubicular regions are clearly present along the dorsal aspects of the hemisphere, but we failed to identify them unequivocally along the caudal and ventral tip of the hippocampus. A temporal portion of the subiculum, if present, differs in its chemoarchitecture from its dorsal counterpart. The perisubicular region, located medially adjacent to the dorsal subiculum may be equivalent to the rat's presubiculum; evidence for the presence of a parasubiculum was rather weak.

Morphology of spiny neurons in the human entorhinal cortex: intracellular filling with lucifer yellow

The present study was designed to investigate the morphology of spiny neurons in the human entorhinal cortex. Coronal entorhinal slices (n = 67; 200 microm thick) were obtained from autopsies of three subjects. Spiny neurons (n = 132) filled with Lucifer Yellow were analysed in different subfields and layers of the entorhinal cortex. Based on the shape of the somata and primary dendritic trees, spiny neurons were divided into four morphological categories; (i) classical pyramidal, (ii) stellate, (iii) modified stellate, and (iv) horizontal tripolar cells. The morphology of filled neurons varied more in different layers than in the different subfields of the entorhinal cortex. In layer II, the majority (81%) of spiny neurons had stellate or modified stellate morphology, but in the rostromedial subfields (olfactory subfield and rostral subfield) there were also horizontal tripolar neurons. Dendritic branches of layer II neurons extended to layer I (94%) and to layer III (83%). Unlike in layer II, most (74%) of the filled neurons in layers III, V and VI were classical pyramidal cells. The majority of pyramidal cells in the superficial portion of layer III had dendrites that extended up to layer II, occupying the space between the neuronal clusters. Some dendrites reached down to the deep portion of layer III. Apical dendrites of layer V and VI pyramidal cells traveled up to the deep portion of layer III.Our data indicate that the morphology of spiny neurons in different layers of the human entorhinal cortex is variable. Vertical extension of dendritic branches to adjacent layers supports the idea that inputs terminating in a specific lamina influence target cells located in various entorhinal layers. There appears to be more overlap in the dendritic fields between superficial layers II and III than between the superficial (II/III) and deep (V/VI) layers, thus supporting the idea of segregation of information flow targeted to the superficial or deep layers in the human entorhinal cortex.

Cellular pathology of hilar neurons in Ammon's horn sclerosis

In addition to functionally affected neuronal signaling pathways, altered axonal, dendritic, and synaptic morphology may contribute to hippocampal hyperexcitability in chronic mesial temporal lobe epilepsies (MTLE). The sclerotic hippocampus in Ammon's horn sclerosis (AHS)-associated MTLE, which shows segmental neuronal cell loss, axonal reorganization, and astrogliosis, would appear particularly susceptible to such changes. To characterize the cellular hippocampal pathology in MTLE, we have analyzed hilar neurons in surgical hippocampus specimens from patients with MTLE. Anatomically well-preserved hippocampal specimens from patients with AHS (n = 44) and from patients with focal temporal lesions (non-AHS; n = 20) were studied using confocal laser scanning microscopy (CFLSM) and electron microscopy (EM). Hippocampal samples from three tumor patients without chronic epilepsies and autopsy samples were used as controls. Using intracellular Lucifer Yellow injection and CFLSM, spiny pyramidal, multipolar, and mossy cells as well as non-spiny multipolar neurons have been identified as major hilar cell types in controls and lesion-associated MTLE specimens. In contrast, none of the hilar neurons from AHS specimens displayed a morphology reminiscent of mossy cells. In AHS, a major portion of the pyramidal and multipolar neurons showed extensive dendritic ramification and periodic nodular swellings of dendritic shafts. EM analysis confirmed the altered cellular morphology, with an accumulation of cytoskeletal filaments and increased numbers of mitochondria as the most prominent findings. To characterize cytoskeletal alterations in hilar neurons further, immunohistochemical reactions for neurofilament proteins (NFP), microtubule-associated proteins, and tau were performed. This analysis specifically identified large and atypical hilar neurons with an accumulation of low weight NFP. Our data demonstrate striking structural alterations in hilar neurons of patients with AHS compared with controls and non-sclerotic MTLE specimens. Such changes may develop during cellular reorganization in the epileptogenic hippocampus and are likely to contribute to the pathogenesis or maintenance of temporal lobe epilepsy.

Morphological characteristics of layer II projection neurons in the rat medial entorhinal cortex

The entorhinal cortex receives inputs from a variety of neocortical regions. Neurons in layer II of the entorhinal cortex originate one component of the perforant path which conveys this information to the dentate gyrus and hippocampus. The current study extends our previous work on the electro-responsive properties of layer II neurons of the medial entorhinal cortex in which we distinguished two categories of layer II neurons based on their electrophysiological attributes (Alonso and Klink [1993] J Neurophysiol 70: 128-143). Here we report on the morphological features of layer II projection neurons, as revealed by in vitro intracellular injection of biocytin. We now report that the two electrophysiologically distinct types of neurons correspond to morphologically distinct types of cells. All neurons (65% of the total cells recorded) that developed sustained, subthreshold, sinusoidal membrane potential oscillations were found to have a stellate appearance. Neurons that did not exhibit oscillatory behavior had either a pyramidal-like (32%) or a horizontal cell morphology (3%). Stellate cells had multiple, thick, primary dendrites. Their widely diverging upper dendritic domain expanded mediolaterally over a distance of around 500 microns close to the pial surface. This mediolateral extent was more than double that of the pyramidal-like cells. Dendrites of stellate cells demonstrated long dendritic appendages, and their dendritic spines had a more complex morphology than those of nonstellates. The stellate cell axons emerged from a primary dendrite and were more than double the thickness (approximately 1.4 microns) of the axons of nonstellate cells. Recurrent axonal collaterization appeared more extensive in axons arising from stellate cells than from pyramidal-like cells.

Cellular distribution in the rat telencephalon of mRNAs encoding for the alpha 3 and alpha 4 subunits of the nicotinic acetylcholine receptor

Pharmacological and electrophysiological studies provide evidence for the involvement of different nicotinic acetylcholine receptor isoforms in rat neocortical and hippocampal signal transduction. Yet, rather little is known on the cellular localization of these isoforms. With the availability of isoform specific nucleic acid probes and sensitive non-isotopic detection systems, nicotinic receptors can be studied on the mRNA level in individual neurons. In this way, we have paradigmatically studied the distribution of the alpha 3 and alpha 4 isoform mRNAs of the nicotinic receptor in the rat telencephalon. In the cerebral cortex, alpha 3 transcripts were mainly located in pyramidal neurons of layers V and VI and in some non-pyramidal cells in layer IV, while alpha 4 mRNA was detected in different types of neurons located in almost all layers. In the hippocampus, local distribution of both transcripts was comparable. Only very few labeled neurons were observed in the dentate gyrus. In the CA region, the specific mRNAs were detected in pyramidal perikarya and individual neurons in the strata oriens and lacunosum-moleculare. Our data show that the applied method is sufficiently sensitive and isoform-selective in order to study the differential expression of nicotinic receptors on the cellular level in the mammalian brain.